Fibula Free Flap, my favorite! Been there, done that. Your worries about standing, walking, exercising were exactly my thoughts as well four years ago. I was back on my feet within a few weeks and walking without difficulty. Standing on one leg is still nearly impossible. On the other hand, my engineering job at the time had me on my feet nine hours per day and walking roughly five miles per day. Like you I worried but found it was not much of an issue.
You likely will have some odd nerve sensations for a year or two in your leg and foot. You also end up with a really cool scar, about 100+ stitches which probably should have a zipper tab tattooed on one end just for laughs. The jaw and mouth will also have nerve issues for a while. For me, I have had issues with my prosthetic teeth rubbing on my ever growing gum tissue. No other issues.
Make sure, and I can't stress this enough, make sure the team that does the Fibula Free Flap surgery has many under their belt, not reading it from a text book. You must ask them and the number hopefully is greater than fifty. This is a relatively newer surgery where just four years ago was done only in a handful of clinics in the US and two in Europe. Mayo, Rochester has trained teams in this procedure from around the world. Perhaps you might consider having this done either at Mayo or a clinic they have worked closely with. It is your life and your body and we hope it lasts a long-long time.
When and where is this scheduled (if you wish to share)?